The present invention relates to a noise reduction circuit in a video apparatus, and more particularly to a circuit for reducing a noise included in a luminance signal input to a luminance signal processing system of a video cassette recorder, a video disc player, or a color television.
A video apparatus such as a color television, a video cassette recorder, or a video disc player needs a luminance signal to control screen brightness. The ratio of the primary colors, i.e., the red, green, and blue luminance signals, is approximately 0.3:0.59:0.11. Mixing the three signals with this ratio is the optimal setting for the color sensitivity of the human eye. However, since such a luminance signal includes noise, a circuit for noise reduction has been developed, which will be briefly described.
FIG. 1 is a conventional circuit for reducing the noise of a luminance signal, in which a noisy input luminance signal as shown in FIG. 2A is filtered in a high-pass filter 1 and results in the waveform shown in FIG. 2B. This signal is then supplied to a limiter 2 which limits its amplitude and results in the waveform shown in FIG. 2C. Thereafter, the amplitude-limited signal is supplied to a subtracter 3 which subtracts the signal shown in FIG. 2C which has passed through limiter 2, from the input luminance signal (FIG. 2A), resulting in the output waveform shown in FIG. 2D, thereby reducing the noise.
However, this noise reduction circuit leaves a noise as shown in FIG. 2D, resulting from a remaining noise and distortion both due to the delay of the signal of filter 1 in the contour portion of the final output signal from subtracter 3. Accordingly, the edge of the luminance signal is collapsed.
Also, a circuit for reducing a noise by clamping a video signal using a horizontal pulse, detecting a noise from the clamped video signal, and controlling the clamping level according to an average level variation of the luminance signal has been known for many years.